Water-in-oil-in-water (W/O/W) multiple emulsions, or double emulsions, are emulsion systems where small water droplets are entrapped within larger oil droplets that in turn are dispersed in a continuous water phase. Multiple emulsions find many applications in industries such as food, pharmaceuticals and cosmetics by enclosing useful substances inside the small water droplets.
Multiple emulsions, either W/O/W or O/W/O emulsions, are generally prepared using a 2-step procedure. For W/O/W emulsions, first, a water-in-oil (W/O) emulsion is formed by blending a first (internal) aqueous phase and an oil phase together in the presence of a suitable oil-soluble (e.g. low hydrophile-lipophile balance (HLB) number) emulsifier. This emulsifier adsorbs to the surface of the water droplets and forms a protective coating that prevents their subsequent aggregation. Furthermore, the emulsifier reduces the interfacial tension between the oil and the water phase, favouring the formation of droplets, and increasing the stability of emulsions. Second, a W/O/W emulsion is formed by homogenizing the W/O emulsion with a second (external) aqueous phase containing a suitable water-soluble (e.g. high HLB number) emulsifier. This emulsifier adsorbs to the surface of the oil droplets and forms a protective coating that prevents their subsequent aggregation. The emulsifier also reduces the interfacial tension between the water and the oil phase, favoring the formation of droplets, and increasing the stability of emulsions. The first step is usually carried out in a high-shear device to produce very fine droplets. The second emulsification step is carried out in a low-shear device to avoid rupturing the multiple droplets.
Several publications relate to the process of preparing W/O/W multiple emulsions:
Fukuda (U.S. Pat. No. 4,254,105) describes a multiple emulsion having a dispersing form of W/O/W and a process for preparation thereof for cosmetics.
McClements et al. (WO 2008/021531) is directed to multi-phase emulsion compositions comprising in a first aqueous phase a biopolymeric gelling component.
Terao et al. (US 2010/0099639) relates to W/O/W multiple emulsion composition containing in an internal aqueous phase an ionic physiologically active substance and a physiologically acceptable compound having a molecular weight of 1,000 or less and generating a polyvalent counterion with two or more valencies for the ionic physiologically active substance.
Rodriquez-Huezo et al. (2004. J Food Science 69(7); E351-E359) is directed to formation of microcapsules containing water and oil soluble carotenoids by spray-drying of W/O/W multiple emulsions.
JP 62244370 describes a W/O/W multiple emulsion comprising an essential oil extracted from citrus fruit peel as the oil phase and an aqueous solution containing water-soluble pigments as the internal aqueous phase.
JP 60 034155 relates to W/O/W type multiple emulsion containing water soluble pigment in an internal aqueous solution.
Due to consumer preferences synthetic food colorants are steadily being replaced by natural ones. However, in comparison with the natural coloring agents, the synthetic coloring agents show greater resistance and stability when exposed to oxidation, changes in temperature, pH and other factors.
Anthocyanins (polyphenolic pigments) are natural, water-soluble, nontoxic pigments displaying a variety of colors from orange to blue. Anthocyanin-rich extracts from fruits and vegetables can be used as food colorants.
Van den Horst and Langelaan (1999) analyzed the separation of the aroma and color compounds of red cabbage concentrate in multiple emulsions.
However, the main drawback in the use of anthocyanins as food colorants is their low stability. In fact, the color stability of anthocyanins depends on a combination of factors: structure and concentration of anthocyanins, pH, temperature, and presence of complexing agents (phenols, metal ions) (Markakis, P. Stability of anthocyanins in foods. In Anthocyanins as Food Colors; Markakis, P., Ed.; Academic Press: New York (1982); pp 163-180.). Anthocyanins are stable red-colored compounds when the pH of the bulk medium is around 4. At pH 6-7, which is typical of many foods including milk products, anthocyanins are no longer red but instead violet-blue. Since the color of anthocyanins changes with pH, it can be challenging to achieve a red color in many pH-neutral foods using anthocyanins.
Phycocyanin is a blue protein complex found in blue-green algae of the cyanobacterium species Spirulina, in particular Spirulina platensis. Phycocyanin is a phycobiliprotein, an oligomeric protein with the linear tetrapyrrole chromophores known as bilins covalently attached to the apoprotein by a thioether linkage. Biliproteins are found assembled in large, distinct granules—phycobilisomes—that are considered analogous to the light-harvesting complexes containing chlorophyll a and b in green plants. The molecular weight and the position and intensity of the absorption maximum of phycocyanin depend on the state of aggregation, which is further influenced by parameters such as solution pH, temperature, protein concentration and algae origin. Phycocyanin is unstable to heat and light in aqueous solution (Jespersen, L, Strømdahl, L. D., Olsen, K., Skibsted, L. H.; Eur Food Res Technol (2005); 220:261-266). Spirulina pigments give an attractive, bright blue color but precipitate at low pH, limiting their usage to beverages and some confectionary applications.
Carminic acid is the free coloring principle extracted from the female cochineal insects Dactylopius coccus costa (Coccus cacti L.). It is a water-soluble natural color that is orange at low pH (where it is most stable), purple at neutral pH, and blue at alkaline pH. Carmine is the aluminum or calcium-aluminum lake of carminic acid on an aluminum hydroxide substrate. It is dispersible but not soluble in water, although it can be dissolved in acidic or alkaline solutions.
Calcium carbonate is a white powder which dissolves at low pH in liquid products and thereby loses its coating properties. This can be a problem in low pH jelly gums where a white product is needed without the use of titanium dioxide. Dissolving of calcium carbonate can also cause confectionery coated with anthocyanins to turn from red to blue by increasing the pH.
Huito pigments are derived from stabilized and concentrated juices of the Genipa americana, an edible fruit. As with the other pigments, double encapsulation can help protect the huito from ingredients in the matrix that might cause its precipitation, and allow the inclusion of antioxidants to protect the pigment.
Carthamus is a dark yellow natural flavoring preparation or natural food ingredient produced by aqueous extraction of carthamus or safflower (Carthamus tinctorius L.). The preparation contains flavorings, pigments, and other ingredients from the plant, so has a typical aroma and flavor profile for safflower concentrate. Double encapsulation can reduce the flavor and aroma profile from this pigment.
The coloring agents mentioned herein are water-dispersible, i.e. they can be readily dissolved or dispersed in an aqueous medium as shown in the Examples herein, which allow for them to be encapsulated in a W/O/W multiple emulsion.
Other water-dispersible pigments not mentioned herein could potentially benefit from an encapsulation technology, for example to control their antioxidant environment.
The natural coloring agents are especially useful as colorants for food products, such as confectionary, fruit preparation, ice cream and dairy. However, their instability at different pH makes use of them difficult.
Additionally, the natural coloring agents may experience fading when subjected to light for an extended time period.
For fruit preparation a specific problem arises when the fruit preparation is layered with yoghurt or white mass; the color from the fruit preparation migrates into the yoghurt.
There is, thus, a continuous industrial need for alternative methods for coloring food products, cosmetics and pharmaceutical products with natural water-dispersible coloring agents as colorant and for methods to render these coloring agents more stable to external factors during use.